Figure 6. Relationships between body mass (BM) and aspects of the digestive physiology of herbivorous vertebrates.

a) wet gut contents [79], [122]; note the similarity in all three vertebrae clades, with a duck species (a flying bird) as a notable outlier; b) dry matter gut contents as calculated from simultaneous passage and digestion studies [16], [81], [82]; note the similarity in the scaling of both measures of gut fill in all three vertebrate clades, with herbivorous birds falling into two categories (flying birds with lower gut fills; flightless or flight-reduced birds such as hoatzin and ostrich with gut fill as in mammals); c) dry matter intake in feeding studies in captivity [16], [81], [82]; note the generally lower intake in reptiles as compared to mammals and birds; a curvature in mammals is evident with a lower scaling in smaller and a steeper scaling in larger species; d) dry matter intake (DMI, on a variety of diets) [16] or organic matter intake (OMI, on a consistent diet) [66] in mammal herbivores >100 kg (no smaller species included in the Foose dataset); note a tendency for a lower scaling in the Foose dataset (see text) that is not significant, raising the question whether the steeper intake scaling in larger herbivores in the Müller et al. dataset is a reaction to a putative decreasing diet quality with increasing BM.